IP Multimedia Subsystem (referred to as IMS) is an IP-based network architecture proposed by the 3rd Generation Partnership Project (referred to as 3GPP). The system builds an open and flexible service environment, supports multimedia applications, and is capable of providing users with abundant multimedia services.
The IMS can be divided into a control layer and a service layer. The control layer mainly includes a Call Session Control Function (referred to as CSCF) for service triggering and control, wherein the CSCF can further be divided into a Proxy CSCF (referred to as P-CSCF), an Interrogating CSCF (referred to as I-CSCF) and a Serving CSCF (referred to as S-CSCF), in which the I-CSCF and the S-CSCF are optional. The service layer mainly includes a series of Application Servers (referred to as AS) for providing specific services, wherein the AS either can be an independent entity, or can exist in the S-CSCF. It works as follows: the control layer (S-CSCF) controls the service triggering based on subscription information of the user, invokes services on the AS, and realizes service functions. Corresponding to a User Equipment (referred to as UE), the AS and the S-CSCF can be uniformly referred to as Server Equipment (referred to as SE).
Meanwhile, there exist in the IMS network a variety of gateway devices for processing IMS signaling and media, such as the P-CSCF and an Access Gateway (referred to as AGW) controlled by the P-CSCF, an Interworking Border Control Function (referred to as I-BCF) and an Interworking Border Gateway (referred to as I-BGW) controlled by the I-BCF, a Session Border Control (referred to as SBC), an Application Layer Gateway (referred to as ALG), etc. These gateways mainly serve to segment a media path, so as to achieve intercommunication between different networks, such as intercommunication between public network equipment and private network equipment, and intercommunication between entities having different encoding functions achieved by transcoding. For the convenience of description, the gateway devices capable of processing IMS signaling and media are uniformly referred to as signaling media gateway, for example, the P-CSCF and the AGW can be regarded as one signaling media gateway.
Since the goal of a session is to achieve media connection, it is required to negotiate media resource information in a session. In the IMS system, the protocol for negotiating the media resource information is a Session Description Protocol (referred to as SDP), and the mode of interaction of the protocol is an offer/answer model, viz. there must be an SDP answer for an SDP offer. The contents of the SDP offer and answer are all media resource information including information of media owner (including address information), media connection address, media type, and information of media port and codec relating to the media type, etc. In the content of the SDP answer, the number, type and sequence of media must be completely identical to those in the SDP offer. The contents of the SDP protocol are carried by a message body of a Session Initiation Protocol (SIP) message, and the process of interaction is independent from the request-response model of the SIP, that is, the request or response message of the SIP may carry an SDP offer, may carry an SDP answer, or may not carry any contents of the SDP protocol.
FIG. 1A is a schematic diagram of the existing IMS signaling and media path and the corresponding signaling flow, and describes the signaling flow during the process in which a terminal UE-A calls a terminal UE-B of the same operator network, and the final media path, wherein the UE-A and the UE-B respectively roam to different visit networks.
For the convenience of description, the figure only shows signaling media gateway entities, other IMS network entities on the call session signaling path are not described, since they only serve to forward the signaling.
As shown in FIG. 1A, S1 is a signaling interaction link between the UE-A and the P-CSCF of the visit network of the calling party, S2 is a signaling interaction link between the P-CSCF of the visit network of the calling party and the ALG of the home network of the calling and called parties, S3 is a signaling interaction link between the ALG of the home network of the calling and called parities and the S-CSCF of the home network of the calling and called parties, S4 is a signaling interaction link between the ALG of the home network of the calling and called parties and the SBC of the visit network of the called party, and S5 is a signaling interaction link between the SBC of the visit network of the called party and the called terminal UE-B. The signaling path between the UE-A and the UE-B is formed by the interconnection of S1-S2-S3-S3-S4-S5.
M1 is a media link between the UE-A and the AGW of the visit network of the calling party, M2 a media link between the AGW of the visit network of the calling party and the ALG of the home network of the calling and called parties, M3 a media link between the ALG of the home network of the calling and called parties and the SBC of the visit network of the called party, and M4 is a media link between the SBC of the visit network of the called party and the UE-B. The media path between the UE-A and the UE-B is formed by the interconnection of M1-M4.
FIG. 1B is a flow chart of the existing establishment process of the IMS signaling and media path. As shown in FIG. 1B, the following steps are mainly included:
step 101, the UE-A initiates a call to the UE-B, for example, sending an INVITE request carrying media resource information of the UE-A, specifically, the media resource information can be carried in the message body, in FIG. 1B, the media resource information is indicated by SDP1; and the request message is transmitted via the path S1;
step 102, the request message passes a signaling media gateway 1, the signaling media gateway 1 forwards the request message after substituting the media resource information in the message with the media resource information allocated by the signaling media gateway 1, in this example, the media resource information allocated by the signaling media gateway 1 is indicated by SDP2; and the request message is transmitted via the path S2;
step 103, the request message passes a signaling media gateway 2, the signaling media gateway 2 forwards the request message after substituting the media resource information in the message with the media resource information allocated by the signaling media gateway 2, in this example, the media resource information allocated by the signaling media gateway 2 is indicated by SDP3; and the request message is transmitted via the path S3, and is forwarded back by the S-CSCF;
step 104, the request message again passes the signaling media gateway 2, the signaling media gateway 2 forwards the request message after substituting the media resource information in the message with the media resource information allocated by the signaling media gateway 2, in this example, the media resource information allocated by the signaling media gateway 2 is indicated by SDP4; and the request message is transmitted via the path S4;
step 105, the request message passes a signaling media gateway 3, the signaling media gateway 3 forwards the request message after substituting the media resource information in the message with the media resource information allocated by the signaling media gateway 3, in this example, the media resource information allocated by the signaling media gateway 3 is indicated by SDP5; and the request message is transmitted via the path S5;
step 106, the request message reaches the UE-B, the UE-B returns a temporary response message, for example, sending a “183 Session Progress” response carrying the media resource information of the UE-B, for example, carrying the media resource information in the message body, in this example, the media resource information of the UE-B is indicated by SDP6; and the response message is transmitted via the path S5;
step 107, the response message passes the signaling media gateway 3, the signaling media gateway 3 forwards the request message after substituting the media resource information in the message with the media resource information allocated by the signaling media gateway 3, in this example, the media resource information allocated by the signaling media gateway 3 at this time is indicated by SDP7; and the response message is transmitted via the path S4;
step 108, the response message passes the signaling media gateway 2, the signaling media gateway 2 forwards the request message after substituting the media resource information in the message with the media resource information allocated by the signaling media gateway 2, in this example, the media resource information allocated by the signaling media gateway 2 is indicated by SDP8; and the response message is transmitted via the path S3, and is forwarded back by the S-CSCF;
step 109, the response message again passes the signaling media gateway 2, the signaling media gateway 2 forwards the request message after substituting the media resource information in the message with the media resource information allocated by the signaling media gateway 2, in this example, the media resource information allocated by the signaling media gateway 2 at this time is indicated by SDP9; and the response message is transmitted via the path S2;
step 110, the response message passes the signaling media gateway 1, the signaling media gateway 1 forwards the request message after substituting the media resource information in the message with the media resource information allocated by the signaling media gateway 1, in this example, the media resource information allocated by the signaling media gateway 1 at this time is indicated by SDP10; and the response message is transmitted via the path S1; at this point, it can be seen that, the UE-A and the signaling media gateway 1 know media resource information of each other, the signaling media gateway 1 and the signaling media gateway 2 know media resource information of each other, by analogy, as a result, the media path between the UE-A and the UE-B is divided into 5 segments, wherein one segment of the media path is from the signaling media gateway 2 to the signaling media gateway 2;
steps 111-114, the UE-B answers the call, sends an answer message, for example, sending a “200 OK”, the message sequentially passes the signaling media gateways 3, 2, 2, 1, and finally reaches the UE-A, and till now, communication is established between the UE-A and the UE-B, and the media path is divided into 5 segments.
It can be concluded from the above description that, if the existing IMS media path establishing method is employed, when a media link is allowed to be established directly between the visit network of the calling party and the visit network of the called party, especially when the calling party and the called party roam to the same place and are served by the same visit network, the media path is lengthened a lot, which wastes equipment resources, and even in some cases, there may appear a media link between a certain signaling media gateway and the signaling media gateway itself.